You can give messages generated in Outlook an expiration date. Once the message has expired, the header will still be visible when viewed in Outlook, however it will have a strikethrough. The message can still be opened, but will give the recipient the indication that the email is no longer relevant.
1. Create a new message.
2. In the Ribbon, go to the Options section on the Message tab.
3. Click the downward pointing arrow in the bottom right corner.
4. In the Message Options window, under Delivery Options, check the Expires after checkbox.
5. Using the corresponding dropdowns, select the date and time that the message will expire.
6. Click Close.